1. Field of the Invention
This invention relates to insulated pipelines, pipeline insulating materials which utilize a bituminous component and particularly to a more cost effective, insulated offshore pipeline.
2. Related Art
At low temperatures, the flow through pipelines can be impeded by high viscosity and wax formation in liquid products such as oil, and by hydrate formation in products such as natural gas. These problems can be reduced by using thermally insulated pipelines, but insulated pipelines are expensive on land and ever more costly offshore. For offshore pipelines it has usually been more cost effective to reduce the need for insulation by injecting various chemicals into the product. Recently, however, more and more oil and gas is being produced in deeper, colder water, from sub-sea production systems where use of viscosity reducing chemicals requires a dedicated line to transport them to the wellhead. This, combined with the fact that the cost of insulating pipelines typically increases with depth, means that insulated pipelines are most expensive where the alternatives are least attractive.
Various materials have been used to insulate land pipelines, including expanded cork, polymer foams, calcium silicate and others. Insulating pipelines offshore is somewhat more complicated because most insulating materials can become saturated in water when submerged. Some insulating materials incorporate watertight closed cell structures, but all have some depth limit at which the cellular structure will collapse, and most will fail in a few hundred feet of water. Furthermore, most common insulating materials have little resistance to impact, abrasion or crushing, and must therefore be encased. If the water depth exceeds the hydrostatic pressure limitations of the material then the casing must also isolate the insulating material from the hydrostatic head of the water.
If the pipeline is laid in sections it is a practical necessity to prefabricate each individual pipe section as an independent pressure vessel. Because pressure resistant double pipes are too stiff to spool, several reel-laid pipelines have been installed with flexible coatings of solid, elastomers or elastomers filled and extended with other lightweight materials. Examples include neoprene and EPDM rubber, EPDM and polyurethane elastomers filled with glass microspheres, and ebonite filled with cork. Unless the insulation requirement is minimal, the total cost of pipelines insulated in this manner is even higher than one which uses a pressure resistant casing to protect less expensive insulating materials.
The thermal resistance offered by paint and corrosion coatings is slight due to the fact that the corrosion coatings are generally thin. Bituminous coatings are commonly used for corrosion coatings for offshore pipelines. "Coat and wrap" coatings comprise two or three layers of kraft paper, felt or fiber glass, that are wrapped onto the pipe as they are being impregnated with hot asphalt or coal tar bitumen that is extended with finely divided mineral fillers such as fly ash, finely divided silica, slate flour or calcium carbonate. These coatings are 0.90 to 0.250 inches thick. "Pipeline mastic" coatings are thicker layers of asphalt concrete extruded onto the pipe. Pipeline mastic comprises calcium carbonate, sand, gravel, fiber glass and asphalt, and is 1/2 to 3/4 inch thick. In both types of coatings the fillers reduce cost and build viscosity, but the effective thermal conductivity of these fillers is five to ten times that of pure bitumen, and they therefore substantially increase the conductivity of the composition. The thermal conductivity of mastic coatings, for example, is 3.5 to 4 times that of pure bitumen. The thermal conductivity of "coat and wrap" coatings is somewhat lower, depending on the fabric wrap, but they are so thin that they provide less insulating value than the naturally occurring phenomenon of "self burial" of the pipeline due to scour and currents.